Weird, and this is with the separate search bar disabled? If I type mario
, then press Enter, then DuckDuckGo opens with the search, and my address bar still says mario
, and I can still change the search engine.
You might want to look for this option under "Search" in the settings:
Show search terms in the address bar on results pages
I'm on Linux as well, so that's probably not it. But I just checked the release notes, and it does say:
So hopefully you'll get it soon!